Porcelain enamel coated products have existed and been used for over three centuries. Porcelain enamel may be defined as a substantially vitreous or glassy, inorganic coating bonded to metal by fusion at a temperature above 800.degree. F. Although there have been many modifications to porcelain enamel technology over the years, the basic principles regarding the formation of porcelain coated products remain the same. Basically, a combination of metals and other organic compounds are combined in the form of a fine powder, like glass, into a frit. A frit may be defined as small friable particles produced by quenching a molten glassy material. The frit is then coated or applied, typically with a liquid carrier such as water, to the prepared surface of a blank or supporting substrate, such as a metal. The frit is then fired, usually at a temperature above 1,000.degree. F., to form the desired porcelain enamel coating on the selected product. Porcelain enamel coated products typically include grills, stove tops, washing machines, plates, utensils, bath tubs, and tags and signs. Because of its outstanding weather, chemical and heat resistance, porcelain enamelling has undergone a recent resurgence in view of a cost aware economy and the need for certain products which exhibit durability over a long period of time.
With respect to outdoor use, porcelain enamel tags and signs offer the ultimate in durability. For this reason, porcelain enamel signs are commonly used for gas pipe lines and electric utility tower transmission signs, which are examples of two applications where it is extremely costly to install a sign and where the durability and extra life of the sign is extremely important. In addition, porcelain enamel signs can be easily cleaned after being defaced, and for this reason, provide an excellent product for use in public areas such as parks, subways, and zoos.
Typically, porcelain signs are currently made using a metal backing such as steel, in view of porcelain enamel's ability to bind to metal. Generally the metal is first pickled and washed in a variety of hot baths, in order to remove any oil or other unwanted material from the surface. If all the oil is not effectively removed from the surface, it will cause a blemish and the porcelain enamel will not adhere to that spot. After pickling the metal blank, it is typically coated with a grey ground coat of frit. This coating may conveniently take place by either dipping the blank in a wet solution of grey frit, or by spraying with a wet solution. The ground coat has been specially made to adhere well to the metal. The ground coat is then fired to form a porcelain enamel coating over the metal backing. The next step is to apply a white (or other solid background color) coating on top of the ground coat. This coating can be painted, sprayed, or dipped over the ground coat as described above in the forming of the ground coat. Following the application of the white coat, the sign blank is then fired again.
After this step, the sign blank is now ready to receive the desired graphics typically in the form of an image and/or words. The image is typically made by an art department by any suitable graphics program available to the art or hand typesetting technique. The image is eventually shot onto a film positive, and the film positive placed on to a previously photo-sensitized, stretched screen, and exposed. The screen now has the image burned into it, and the image appears as openings in the screen mesh; with the unprinted background having the opaque coating.
The finished screen is then positioned in a screen press, and porcelain enamel ink is loaded into the screen, and the white coated porcelain enamel blank sign is printed. The porcelain enamel ink is available from a number of frit manufacturers of powdered frits and oils. The oils help the frit flow and make the frit screen printable.
Once the words are printed on the sign, generally in black, the frit is allowed to air dry. During the drying process, the oil in the frit evaporates, and after the black words dry, the sign is then fired. After cooling, the sign is then completed.
The above process represents the current state of the art for making porcelain enamel signs and tags. This process, although suitable for a large run of signs or hags, is extremely expensive for making one-of-a-kind or small numbers of signs due to the labor intensive costs involved in the various steps. Therefore, there has been a continuing need in the field for an improved process, which would allow for porcelain enamel signs to be made more simply and efficiently.